Generally speaking, in third generation systems, one objective is to improve performance, and in particular to increase capacity and/or to improve quality of service.
One technique widely used is known as power control, and in particular as closed loop power control.
The objective of closed loop power control is to maintain a parameter representative of the transmission quality on each link between a base station and a mobile station, for example the signal-to-interference ratio (SIR), as close as possible to a target value. The mobile station periodically estimates the SIR in the downlink direction, i.e. in the direction from the base station to the mobile station, for example, and compares the estimated SIR to the target SIR. If the estimated SIR is less than the target SIR, the mobile station requests the base station to increase its transmission power. On the other hand, if the estimated SIR is greater than the target SIR, the mobile station requests the base station to reduce its transmission power.
The target SIR is an important parameter in such systems. If the target SIR is set at a value greater than that which is strictly necessary, the level of interference in the system is increased unnecessarily, and the performance of the system is therefore degraded unnecessarily; on the other hand if the target SIR is set at a value less than that which is strictly necessary, the quality of service on the link in question is degraded.
The target SIR is generally chosen as a function of the required quality of service and is routinely adjusted by an external loop algorithm (as opposed to the algorithm previously referred to, which is an internal loop algorithm). The principle of the external loop algorithm is to estimate the quality of service regularly and to compare the estimated quality of service with the required quality of service. The quality of service is generally represented by a bit error rate (BER) or a frame error rate (FER) for voice services or by a block error rate (BLER) for packet-mode data services. If the estimated quality of service is lower than the required quality of service, the target SIR is increased; if not, the target SIR is reduced.
Unlike the internal loop algorithm, which must be relatively fast to track variations in the SIR as closely as possible, the external loop algorithm must be relatively slow, because the quality of service must be averaged over a period to obtain a reliable estimate. In systems like the UMTS in which information transmitted is structured in frames which are in turn structured in time slots, the SIR of the received signal is typically estimated and compared with the target SIR in each time slot of a frame and the quality of service is averaged over several frames.
The relative slowness of the external loop algorithm can cause problems, however, especially if the required quality of service changes, for example:                in the event of a change of transmission mode from an uncompressed mode to a compressed mode, or vice versa,        in the event of a change of the service required (in particular a change of transmission bit rate),        in the event of a change of transmission bit rate for a given required service (for example for packet-mode data services),        in the event of a change in environmental conditions (for example a change in the speed of the mobile, a change in radio propagation conditions, etc.),        etc.        
In the following description, the emphasis is more particularly on control problems caused by using the compressed mode.
In a system like the UMTS, for example, the compressed mode has been introduced in the downlink direction to enable a mobile station, also referred to as a user equipment (UE), to perform measurements on a frequency different from its uplink transmission frequency, and essentially consists of stopping transmission in the downlink direction for the duration of a predetermined transmission gap. This is outlined in FIG. 1, which applies to the situation in which the information transmitted is structured in frames and shows a series of successive frames including compressed frames (for example the frame T1) and uncompressed frames (for example the frame T2).
The instantaneous bit rate is increased in a frame which has been compressed by increasing the coding rate or by reducing the spreading factor, and the target SIR must therefore be increased in approximately the same proportion.
Also, as closed loop power control is no longer active during a transmission gap, performance is significantly degraded, mainly, as the applicant has found, during a compressed frame and during one or more frames referred to as “recovery frames” following the compressed frame. The degradation can be as much as several decibels. To retain the same quality of service as in the normal (uncompressed) mode, this effect must also be compensated by increasing the target SIR during these frames.
However, because the external loop algorithm is relatively slow, several frames will probably be necessary before changing the target SIR correspondingly, and the target SIR may even be increased just after the compressed or recovery frames, at a time when the increase is no longer required, which degrades performance in all cases.
European Patent Applicant No. 99401766.3 filed 13 Jul. 1999 by the applicant proposes a solution that avoids degraded performance in compressed mode.
Briefly, the basic idea of the earlier application is to anticipate the target SIR variation, i.e. to apply a corresponding variation ΔSIR to the target SIR in advance.
Another idea set out in the earlier application is to separate the increase in the target SIR due to the increase in the instantaneous bit rate and the increase δSIR in the target SIR due to degraded performance in compressed frames, i.e. due to transmission gaps.
For the downlink direction, for example, since the user equipment knows the bit rate variation, only the increase δSIR in the target SIR due to degraded performance in compressed frames has to be signaled to the user equipment by the network. The additional signaling resources needed can be small if the variation is signaled with other compressed mode parameters, including the duration of the transmission gaps, their period, etc.
The user equipment can increase the target SIR by ΔSIR just before the compressed frame or just after transmission of the compressed frame is interrupted and reduce it by the same amount just after the compressed frame. This target SIR variation is added to the conventional external loop algorithm, which must take it into account.
Another idea set out in the earlier application is that performance in the recovery frames can also be degraded because of the interruption in power control during the transmission gap, at least when the transmission gap is at the end of a compressed frame. It would therefore also be desirable to increase the target SIR during the recovery frames and to signal this target SIR increase to the user equipment. Alternatively, to reduce the amount of signaling needed, the same δSIR value could be used as for the compressed frames.
Thus, according to the earlier application, anticipating the target SIR variation during compressed and recovery frames increases the efficiency of the external power control loop in the compressed mode.
Another idea set out in the earlier application is for the user equipment to simultaneously increase its transmission power in the same proportion before the compressed frame and likewise reduce it in the same proportion after the compressed frame. This avoids problems caused in particular by the step mode operation of the internal loop algorithm and the new target SIR value is therefore reached faster (if the target SIR variation is 5 dB and if the power control step is 1 dB, then the conventional internal loop algorithm would require five time slots to reach the new target value, for example).
Thus, according to the earlier application, additionally anticipating the transmission power variation also increases the efficiency of the internal power control loop in the compressed mode.
A problem can nevertheless arise in obtaining an anticipated variation of the transmission power corresponding to the target SIR variation. Because in practice the entity of the system in charge of determining and/or applying the anticipated variation of the transmission power is not necessarily the same as the entity of the system in charge of determining and/or applying the target SIR variation, the variations determined and/or applied in this way by the different entities can be different, and performance can then be degraded.
Generally speaking, and as outlined in FIG. 3, a mobile radio system includes the following entities: mobile stations, for which the UMTS term is “user equipment” (UE), base stations, for which the UMTS term is “B node”, and base station controllers, for which the UMTS term is “radio network controller” (RNC). The combination of the B nodes and the radio network controllers is called the UMTS terrestrial radio access network (UTRAN).
The external power control loop is generally in the receiver, in the downlink user equipment, for example, because it is more logical to estimate the required quality of service (BER, FER, BLER, etc.) using the external loop in the receiver. The receiver then knows the target value variation ΔSIR. On the other hand, the anticipated variation of the transmission power must be applied in the transmitter, in the downlink B node, for example, and must therefore also be known to the sender.
Also, in a system like the UMTS, the radio network controller is responsible for network control and for controlling the actions of the user equipment, and the B node is principally a transceiver. The uplink external power control loop is therefore in the radio network controller. The internal power control loop is partly in the user equipment and partly in the B node; for example, for transmission in the uplink direction, the B node compares the estimated SIR with the target SIR and sends a power control command to the user equipment. The user equipment modifies its transmission power as a function of power control commands sent by the B node. The downlink external power control loop is in the user equipment (some parameters needed to determine ΔSIR, such as the parameter δSIR previously referred to, are signaled to the user equipment by the radio network controller). For this reason, the B node does not know the value of ΔSIR for the downlink direction, including the component δSIR signaled to the user equipment by the radio network controller. It knows only the value ΔSIR for the uplink direction.
For the downlink direction, one solution to this problem would be for the radio network controller to signal the parameter δSIR needed to determine the target SIR variation not only to the user equipment but also the B node.
However, this kind of solution has the disadvantage of significantly increasing the amount of signaling that has to be exchanged and therefore of not using the available transmission resources efficiently.
There is therefore a requirement for a solution that could avoid such drawbacks or, more generally, that could reduce the amount of signaling required without degrading performance.
In a system like the UMTS in particular, different channels called “dedicated physical channels” can be transmitted simultaneously on the same physical channel.
There are two types of dedicated physical channel:                dedicated physical data channels (DPDCH), and        dedicated physical control channels (DPCCH).        
Each user equipment in connected mode is allocated a DPCCH and one or more DPDCH, as required.
In the downlink direction, for example, the DPDCH and the DPCCH are time-division multiplexed in each time slot of a frame, as shown in FIG. 2.
As also shown in FIG. 2, the DPCCH includes three fields:                a Pilot field containing a pilot signal enabling the mobile station to remain synchronized with the network and to estimate the propagation channel,        a transmit power control command field TPC containing power control command bits to be used by the internal power control loop, and        a transport format combination indicator field TFCI containing transport format indicator bits which, indicate the transport format used for each DPDCH, including in particular the coding, interleaving, etc. scheme, which depends on the corresponding service.        
As described in section 5.2.1.1 of the document 3G TS 25.214 V3.2.0 (2000-03) published by 3GPP (“3rd Generation Partnership Project”), the power control algorithm simultaneously controls the power of the DPCCH and the DPDCH and the transmission power of each of the TFCI, TPC and Pilot fields is offset relative to the transmission power of the DPDCH by a respective offset PO1, PO2, PO3 determined by the network.
However, problems can arise if this technique is used in combination with the technique of anticipating the variation of the transmission power, as described in the earlier application previously referred to, but it was not the main aim of the earlier application to solve those problems. In particular, the transmission power for at least one of the fields of the DPCCH can momentarily become greater than would strictly be necessary, leading to an unnecessary increase in the level of interference in the network and/or an unnecessary reduction in network capacity, together with an unnecessary increase in power consumption in the sender concerned.
There is also a need for a solution that could avoid such problems, or more generally that could obtain optimum anticipated variations of the transmission power for each field or channel.